Various thermal transfer methods have hitherto been known in the art. Among others, a method has been proposed which comprises: providing a sublimable dye as a recording agent; supporting the sublimable dye on a substrate sheet, such as paper or a plastic film, to prepare a thermal transfer sheet; and transferring dyes from the thermal transfer sheet onto an image-receiving sheet, such as paper or a plastic film provided with a dye-receptive layer, to form various full-color images. In this case, a thermal head in a printer is used as heating means, and a large number of color dots of three or more colors are transferred onto an image-receiving sheet by heating for a very short period of time to reproduce a full-color image of an original by the large number of color dots.
The image-receiving sheet usable for the formation of images by the above method is limited, for example, to plastic sheets dyeable with dyes, or papers with a dye-receptive layer being previously provided thereon, and images cannot be formed directly on conventional plain papers or the like. It is a matter of course that, even in the case of conventional plain papers, for example, postcards, memo papers, letter papers, and report pads, image formation is possible when a dye-receptive layer is previously formed on the surface of these papers. The commercial production and sale of such a wide variety of image-receiving papers with a dye-receptive layer previously formed thereon, however, incur high cost, and, thus, the sale of them as an established image-receiving sheet is difficult. In order to solve this problem, the use of a dye-receptive layer transfer sheet has been proposed wherein, when the formation of an image on existing sheet products, such as plain papers, is contemplated, a dye-receptive layer can be simply formed only on a necessary portion(s) of the sheet (see, for example, Japanese Patent Laid-Open No. 264994/1987). Another known method for simplifying the procedure is to use a composite thermal transfer sheet. In this method, yellow, cyan, magenta, and optionally black dye layers are formed in a face serial manner on a plane of a continuous substrate sheet, and a transferable dye-receptive layer is further provided on the same plane of the substrate sheet. At the time of transfer, the dye-receptive layer is first transferred on an image-receiving sheet, and, subsequently, various dyes are transferred onto the transferred dye-receptive layer to form a full-color image. Further, a composite thermal transfer sheet, wherein a protective layer for protecting the formed dye image has been further formed in a face serial manner, has also been proposed.
Thus, as described above, a dye image is thermally transferred onto a receptive layer in a dye image-receiving sheet with a receptive layer being previously formed thereon, or onto a receptive layer in the thermal transfer dye image-receiving object produced by transferring a receptive layer from a receptive layer transfer sheet onto an object. Further, the thermal transfer of a protective layer on the receptive layer with a dye image being formed thereon has been extensively carried out in the art in order to enhance fastness properties of prints formed by thermal transfer, such as abrasion resistance and lightfastness.
In forming a protective layer by thermal transfer on a receptive layer, with a dye image formed thereon, provided directly on the above image-receiving paper, or on a receptive layer, with a dye image formed thereon, in an image-receiving paper produced by transferring a receptive layer from a receptive layer transfer sheet onto a paper, however, many troubles occur including that, due to a release agent added for providing satisfactory releasability, such as a silicone oil or a phosphoric ester or fluoro-surfactant, the adhesion between the receptive layer and the protective layer is so low that the protective layer cannot be evenly transferred onto a predetermined region and, consequently, a high level of unevenness occurs, or otherwise, the protective layer is easily separated from the receptive layer during handling of prints.
The use, in the receptive layer, of a resin having relatively good releasability from the transfer sheet, such as polystyrene, cellulose acetate, or polyolefin, is also considered as means for reducing these troubles. These resins, however, cannot be substantially used because, as compared with polyesters, vinyl chloride-vinyl acetate copolymers and other resins commonly used in the art, they have lower dyeability (sensitivity) and are not substantially dyed.
Further, in recent years, shortening the printout time or energy saving, that is, power reduction, in the printout through printing at low energy has been demanded in the art. One means effective for printing at low energy is to increase the sensitivity of the receptive layer to permit the receptive layer to be more easily dyed with the transferred dye. Japanese Patent Laid-Open No. 274990/1986 describes that various plasticizers are added to increase the sensitivity of the receptive layer. Plasticizers having a molecular weight of not more than about 600, however, have a problem that they are likely to migrate within the receptive layer and cause a change in sensitivity according to a storage environment in which the dye-receptive layer is stored before printing. On the other hand, for plasticizers having a molecular weight of not less than 600, the larger the molecular weight, the better the stability. Therefore, the influence of the environment in which the dye-receptive layer is stored before printing is small. However, as the molecular weight is increased, that is, as the stability is increased, the compatible resin usable for a receptive layer is limited, or otherwise satisfactory plasticization effect cannot be attained. Further, since the plasticization of the resin for a receptive layer softens the resin, the resin is often likely to fuse to the transfer sheet. For this reason, the amount of the release agent required is larger than that in the case of the base resin alone without plasticizer loading. This causes the above-described troubles caused at the time of the adhesion of the protective layer to the receptive layer to become more serious.
Accordingly, it is an object of the present invention to provide a receptive layer which is satisfactorily releasable from a dye and a dye binder, can form good images by sublimation dye transfer with high sensitivity, and permits a protective layer to be normally transferred onto the receptive layer of prints.